VM_PHYSSEG_MAX

Hi, to get -current and netbsd-6 to boot on recent IBM hardware, I need to
bump VM_PHYSSEG_MAX in sys/arch/i386/include/vmparam.h and
sys/arch/amd64/include/vmparam.h to > 10 (I've used 16 for the purposes of
testing). As this only sizes a small array, I don't see a problem with
doing this, but people who are more knowledgeable in this area might have
an opinion.

I would like to commit this ASAP to ensure the tweak is in the 6.0 release
even if we later look at the memory initialisation in more detail with
DEBUG_MEMLOAD.